A metal-oxide semiconductor field-effect transistor (MOSFET) is a type of field-effect transistor that functions by electronically varying the width of the MOSFET channel along which charge carriers flow. The wider the MOSFET channel, the better the MOSFET can conduct. MOSFETs include gate, drain and source components. Charge carriers enter the channel via the source, and exit via the drain. The width of the MOSFET channel can be controlled by varying the voltage that is placed onto a gate electrode. In conventional MOSFETs the gate electrode is generally insulated from the channel by a thin layer of oxide.
MOSFET operational parameters affect the operation and performance of the MOSFET. MOSFET operational parameters include drain-source breakdown voltage (BVds) and drain-source on resistance (RDSon).
The MOSFET BVds is the minimum voltage that causes a portion of an insulator to become electrically conductive. Thus, generally a high BVds is desirable. Importantly, when the BVds is exceeded, current flow can occur which can prevent the MOSFET from shutting off properly. RDSon is the drain-source resistance at a specified drain current and gate-source voltage. In many applications a low RDSon is desirable and is associated with an increased MOSFET current carrying capability.
MOSFET designers often make tradeoffs between BVds and RDSon. For example, increasing the BVds by incorporating a thicker and lower doped drift region results in a higher RDSon. However, lowering RDSon by incorporating a thinner and higher doped drift region decreases BVds. Accordingly, by considering tradeoffs, designers seek to find the optimal BVds and RDSon for a MOSFET. Due to different trench widths used in the active and the trench edge termination area, it is difficult to achieve similar BVds both on the active area and on the edge termination area.
FIG. 1A shows a conventional MOSFET that includes active 101 and edge termination 103 areas. As is illustrated in FIG. 1A, the desired direction of current flow is vertical through the MOSFET (see dotted line representing the vertical channel 105 next to active area trench 107). However, if BVds is exceeded, then breakdown can occur in the oxide that lines the corners of device trenches, and undesirable current flow can occur in the MOSFET. This is because many conventional MOSFETs exhibit uneven electric fields where the strength of the electric field can be greatest at corners of MOSFET trenches.
FIG. 1B shows trench locations 111, 113 and 115 that are vulnerable to breakdown in the oxide that lines the walls of the edge termination area trenches 109 of the conventional MOSFET shown in FIG. 1A. As discussed above, such current flow can prevent a MOSFET from shutting off properly. Importantly, many conventional MOSFETs are provided with inadequate protection against edge termination area voltage breakdown and are susceptible to such current flow.